This invention relates to wireless communication systems and, more particularly, to methods of assigning channel codes in such communication systems.
Wireless communication systems have been developed to allow transmission of information signals between an originating location and a destination location. Both analog (first generation) and digital (second generation) systems have been developed to transmit information signals over communication channels linking the source and destination locations. Digital methods tend to afford several advantages over analog systems. For example, improved immunity to channel noise and interference, increased capacity, and encryption for secure communications are advantages of digital systems over analog systems.
While first generation systems were primarily directed to voice communication, second generation systems are required to support both voice and data applications. Numerous techniques are known in second-generation systems for handling data transmissions that have different transmission requirementsxe2x80x94data transmission being typically of relatively short duration, as compared to voice transmission, and usually not requiring continuous access to the communication channel. Several modulation/coding arrangements have been developed, such as frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA), to increase the number of users that can access a wireless network. CDMA systems are more immune to multiple path distortion and co-channel interference than FDMA and TDMA systems and reduce the burden of frequency/channel planning that is common with FDMA and TDMA systems.
In a CDMA system, a binary code sequence (i.e., channel code) is assigned to each active user within a cell to uniquely identify the user and spread the user""s signal over a larger bandwidth. Multiplied by the assigned code, the user""s signal is spread over the entire channel bandwidth, which is wider than the user signal bandwidth. The ratio of the system channel bandwidth to the user""s bandwidth is the xe2x80x9cspreading gainxe2x80x9d of the system. The capacity of the CDMA system is proportional to the xe2x80x9cspreading gainxe2x80x9d for a given signal-to-interference (S/I) level. After reception of the transmitted signal, the signal of each user is separatedxe2x80x94i.e., de-spread, from the signals of other users by using a correlator keyed to the code sequence of the desired signal.
First-generation analog and second-generation digital systems are designed to support voice communication with limited data communication capabilities. Third-generation wireless systems, using wide-band channel management technologies, are expected to effectively handle a large variety of services, such as voice, video, data and imaging. Among the features that will be supported by third-generation systems is the transmission of high-speed data between a mobile terminal and a land-line network. As is known, high-speed data communications is often characterized by a short transmission xe2x80x9cburstxe2x80x9d at a high data transmission rate followed by some longer period of little or no transmission activity from the data source. To accommodate the burst nature of such high-speed data services in third-generation systems, it is necessary for the communications system to assign a large bandwidth segment (corresponding to the high data rate) from time to time for the duration of the data burst. With the ability of the third generation systems to handle such bursty high-speed data transmission, throughput and delay for users can be advantageously improved. However, because of the large amount of instantaneous bandwidth required for transmission of a burst of high-speed data, the management and, particularly the allocation, of channel codes thereto, must be handled with care to avoid unwarranted interference with other services using the same frequency allocation.
In a typical wireless communication system, for example, IS-95, a voice user may have an assigned code sequence of 64 chips per symbol, however, because of time and bandwidth constraints, a typical data user may require a code sequence that has a significantly fewer number of chips per symbol. Hence, in the next generation communication systems assignment of codes of different lengths is necessary.
The invention described herein is a method to manage the assignment of channel codes of different lengths while maintaining an orthogonal relationship among the assigned codes. By selectively assigning longer length codes in the presence shorter-length codes or shorter-length channel codes in the presence of longer-length channel codes, channels codes of different lengths may be assigned in an order that avoids conflicts and maximizes the number of channel codes available for short-length code assignments (i.e., high data rate transmission).
A novel method of managing the selection of channel codes of different lengths from the available channel codes is disclosed. The methodology first distinguishes between transmissions that may use relatively long length channel codes,xe2x80x94e.g., voice and low data rates, and those that must use relatively short length channel codes. Long length channel codes are selected using one scheme to access a list of channel codes, while shorter-length channel codes are selected using a second scheme to access the same list of channel codes. More specifically, in accordance with one method of the invention, longer-length channel codes are selected from one end of a list of available channels codes while shorter-length channel codes are selected from an opposing end of the same list of channel codes. Accordingly, a larger number of channel codes remain grouped together for assignment to short-length code transmissions.
Further, to avoid conflicts and maintain orthogonality among the assigned codes, shorter-length channel codes are assigned after confirming that those longer-length channel codes that depend from a proposed shorter length channel code are available for assignment. If the dependent longer-length channel codes are not available for assignment then the proposed shorter-length channel code is also not available for assignment. Limiting shorter-length channel codes to channel codes in which dependent longer-length channel codes are available insures that a proposed shorter-length channel code sequence remains orthogonal to different length channel codes that are already assigned.